Surface treatment of semiconductor substrates

ABSTRACT

Surface cleaning, chemical treatment and drying of semiconductor substrates is carried out using foam as a medium instead of a condensed phase liquid medium. In cleaning and chemical treatment, by introducing a foam into an overflow vessel the foam is caused to pass over the substrate in moving contact therewith. Drying of the substrate is carried out, using a water solution of carbon dioxide in a pressurizable vessel. By releasing the pressure in the vessel, a layer of foam is established on the surface of the solution. The solution is discharged from the vessel, causing the foam layer to pass over the substrate in moving contact therewith. The carbon dioxide reduces the surface tension of the water, thereby enabling the foam layer to be produced and also assisting in the elimination of water from the surface of the substrate. In both cases, the use of foam reduces materials requirements and also reduces the quantity of particles deposited onto the substrate in the treatment process.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation of application Ser. No. 208,112, filedDec. 9, 1998, now patent No.

BACKGROUND OF THE INVENTION

[0002] This invention relates generally to the treatment of the surfacesof devices, especially semiconductor wafers and other electronic orelectro-optical devices, at various stages of production. It relatesmore particularly to novel cleaning, chemical treatment and dryingprocesses in which, instead of a condensed phase medium (liquid), a foamis used as a medium for the various operations such as cleaning,etching, neutralization and drying.

[0003] Semiconductor cleaning, chemical treatment and drying technologyis well developed. Examples of known processes are those described inU.S. Pat. No. 4,781,764, dated Nov. 1, 1988, U.S. Pat. No. 4,911,761,dated Mar. 27, 1990, U.S. Pat. No. 5,271,774, dated Dec. 21, 1993 U.S.Pat. No. 5,656,097, dated Aug. 12, 1997 and U.S. Pat. No. 5,571,337,dated Nov. 5, 1996. However, cleaning, chemical treatment and drying ofsemiconductors is very expensive. Moreover, performance requirementswill soon exceed the present and expected capabilities of currentcleaning techniques.

[0004] Current processes for the cleaning and treatment of semiconductorwafers and other electronic devices have several serious drawbacks fromthe standpoint of cost, safety and effectiveness.

[0005] High purity deionized water is typically used as a solvent.However, achieving the necessary high purity levels is very expensive.Indeed, all phases of the cleaning operation, including purchasing,transportation, storage, internal distribution, consumption, anddisposal, are expensive.

[0006] Most of the substances used in the cleaning and chemicaltreatment processes, such as fluorides, solvents, acids, heavy metals,oxidizers, etc., are toxic, flammable, or otherwise hazardous orobnoxious.

[0007] Chemical treatment and cleaning operations are also major sourcesof chemical contamination of the final product. Such contaminationresults from errant surface reactants, and physical contamination byundesired, very small solid particles. These very low levels ofcontaminants are delivered to the product, in part from the chemicaltreating and cleaning materials themselves, even though they areultrapure. They are also delivered to the product from fittings, piping,tanks, valves, and other components of storage and delivery systems.

[0008] Contaminants on semiconductor wafer surfaces exist as films,discrete particles or groups of particles and adsorbed gases. Surfacefilms and particles can be classified as molecular compounds, ionicmaterials and atomic species. Molecular compounds are mostly particlesor films of condensed organic vapors from lubricants, greases, photoresists, solvent residues, organic components from deionized water orplastic storage containers, and metal oxides or hydroxides. Ionicmaterials comprise cations and anions, mostly from inorganic compoundsthat may be physically adsorbed or chemically bonded, such as ions ofsodium, fluorine and chlorine. Atomic or elemental species comprisemetals, such as gold and copper, which may be chemically bonded to thesemiconductor surface, or they may consist of silicon particles or metaldebris from equipment.

[0009] Semiconductor devices, especially dense integrated circuits, arevulnerable to all of these contamination sources. The sensitivity is dueto the small feature sizes and the thinness of the deposited layers onthe wafer surface. These dimensions are in the submicron range. Thesmall physical dimensions of the devices make them very vulnerable toparticulate contamination in the air, from workers, generated by theequipment, and present in processing chemicals. As the feature size andfilms become smaller, the allowable particle size must be controlled tosmaller dimensions. In general, the particle size should be 10 timessmaller than the minimum feature size. Currently, the minimum featuresize for commonly available semiconductor chips is 0.25μ, thereforesuggesting particle control to 0.025μ.

[0010] Conventional cleaning technologies, utilizing condensed phasesolutions, when properly applied, remove a majority of the contaminantsgenerated during the chemical processing of the semiconductor wafers.Liquid systems currently in use can delivery satisfactory results, andacceptable product can be produced. However, the current trend is torequire the chemical and equipment suppliers to provide increasinglyclean performance. Equipment and chemical suppliers are facingtremendous performance challenges as the feature size decreases. At thesame time, semiconductor manufacturers do not want their costs toincrease.

[0011] Another problem addressed by this invention is the drying ofsurfaces in the production of semiconductor wafers and similar devices.

[0012] Semiconductor wafers are not manufactured in a continuousprocess. Since there are many semiconductor wafer configurations,batches of wafers are processed through certain steps, and then stored.Later the batches are subjected to additional processing steps, andagain stored. The processing and storage sequence may be repeatedseveral times before processing is completed.

[0013] In general, at the end of each process sequence, thesemiconductor wafers are dried, often even when the next step willproceed almost immediately. Wafers can be transported from one processsequence to the next only after they have been dried, and they can onlybe stored safely when they are dry. Therefore, the drying process iscarried out frequently in the processing of a given wafer, and is veryimportant.

[0014] Recently, isopropyl alcohol has become a preferred dryingsolvent. A variety of processes have been developed and commercializedusing isopropyl alcohol either hot or cold, and as a vapor, a liquid ora combination of vapor and liquid. Semiconductor wafer producers havebeen moving toward reduced isopropyl alcohol usage because of its cost,fire hazards, disposal problems, and VOC (volatile organic compound)emissions.

[0015] U.S. Pat. No. 4,911,761, dated Mar. 27, 1990, describessemiconductor wafer processing in which various fluids passed overwafers in fixed positions. The drying sub-system utilizes superheatedisopropyl alcohol vapor generated in a distillation apparatus.

[0016] U.S. Pat. No. 5,271,774, dated Dec. 21, 1993 describes atechnique for removing water from a semiconductor wafer using low levelsof solvents such as isopropyl alcohol, applied as a vapor, to reduce thesurface tension of a film of liquid on the substrate, and thereby reducethe quantity of material remaining on the surface of the substrate. Acentrifuge is used to facilitate the removal of the liquid film.

[0017] U.S. Pat. No. 5,571,337 describes another technique for dryingsemiconductor wafers, utilizing a trace amount of a polar organiccompound in a carrier gas composed of oxygen, nitrogen, argon, ormixtures. This patent describes the drying of wafers without the use ofisopropyl alcohol, using only warm nitrogen gas. Thus, the industry hasproceeded from the use of large quantities of isopropyl alcohol, tominimal quantities of isopropyl alcohol, and then to processes which useno isopropyl alcohol at all.

[0018] The principal objects of this invention are to increase theeffectiveness of chemical treatment, cleaning and drying operations, andto reduce the cost of such operations. Further objects of the inventionare to improve the safety of the chemical treatment, cleaning and dryingoperations and to reduce the discharge of hazardous or obnoxioussubstances from the treating and cleaning operations.

SUMMARY OF THE INVENTION

[0019] Briefly, the invention takes advantage of a desirablecharacteristic of foam, namely that, from a volumetric standpoint, agiven quantity of foam consists mostly of gas. Therefore the quantity ofsmall particles delivered to a substrate by the liquid component of thefoam is much smaller than the quantity of particles delivered to asubstrate by an equivalent volume of a liquid.

[0020] The expansion ratio of foam, i.e. the volume of the foam dividedby the volume of its liquid component, defines the cost and performancebenefit available from the use of foam. For example, if the expansionratio is 10, the volume of liquid is reduced to {fraction (1/10)} of thevolume of liquid to which the substrate is exposed in a condensed phasechemical treatment or cleaning step. This not only achieves atheoretical materials cost reduction of 90% but also reduces thequantity of contaminating particles delivered to the substrate by afactor of 10. A small increase in the expansion ratio results in arelatively large overall benefit. Additional cost savings can beobtained as a result of the reduction in the volume of the cleaningmedium. The resulting smaller inventory of cleaning medium, and the sizereductions that can be achieved in components such as tanks, valves,pipes, pumps, etc., lead to lower floor space requirements, which, forsemiconductor fabrication facilities, is very significant.

[0021] In accordance with one aspect of the invention, the treatment,i.e. cleaning or chemical treatment, of a semiconductor substrate iscarried out by the steps of generating a foam consisting of gas bubblesand a liquid component, and causing the foam to pass over the substratewhile in moving contact therewith.

[0022] In accordance with another aspect of the invention, asemiconductor substrate is dried by a process comprising the steps ofgenerating a foam consisting of carbon dioxide bubbles and deionizedwater, and causing the foam to pass over the substrate in moving contacttherewith.

[0023] The cleaning or chemical treatment steps, and the drying step canbe carried out in the same treatment vessel. The cleaning, chemicaltreatment and drying steps can be carried out in sequence withoutremoving the substrate from the treatment vessel.

[0024] Other features of the invention are applicable not only to thecleaning, chemical treatment and drying of semiconductor substrates, butalso to the cleaning, chemical treatment and drying of other substrateswhere the removal of extremely small particles or other contaminants, oravoidance of their deposition onto the substrate, is important.

[0025] Preferably, the semiconductor substrate is supported in a foamtreatment vessel, the foam is generated outside the foam treatmentvessel and introduced into the foam treatment vessel, and foam incontact with the semiconductor substrate is caused to pass over thesubstrate as a result of its displacement by foam introduced into thefoam treatment vessel. A sufficient quantity of foam may be introducedto fill the treatment vessel and thereafter, by continued introductionof foam into the treatment vessel, foam may be caused to discharge fromthe treatment vessel.

[0026] For cleaning the substrate, the liquid component of the foam mayconsist of a surface tension-reducing agent and deionized water, and themovement of the foam removes particles from the substrate. For chemicaltreatment of the substrate, the liquid component of the foam may includeone or more reactants, such as ammonium hydroxide, hydrofluoric acid,nitric acid, etc., so that a chemical reaction takes place between thesubstrate and the reactant as the foam passes over the substrate. Thereactants themselves may serve as surface tension-reducing agents.However, ordinarily, since the reactants will be present in insufficientconcentrations to produce adequate quantities of foam, conventionalsurfactants or other additional surface tension-reducing agents will beincluded along with the reactants.

[0027] In a preferred embodiment, the foam introducing step is carriedout by first introducing a foam consisting essentially of a surfacetension-reducing agent and deionized water, thereafter introducing afoam comprising deionized water, a surface tension-reducing agent and atleast one reactant for chemical treatment of the substrate whereby achemical reaction takes place between the substrate and the reactant,and thereafter introducing a foam consisting essentially of a surfacetension-reducing agent and deionized water, whereby said at least onereactant is rinsed from the substrate.

[0028] A foam-based cleaning system in accordance with the invention caneffectively substitute for a liquid phase system using sonic energy.

[0029] The process may be carried out by alternately introducing foamconsisting essentially of a surface tension-reducing agent and deionizedwater, and foam comprising a surface tension-reducing agent, deionizedwater and at least one reactant for chemical treatment of the substrate.By carrying out the process in this manner, a series of chemicaltreatment steps can be carried out on a semiconductor substrate, allusing foam as a vehicle for the reactants.

[0030] In accordance with another aspect of the invention, for dryingthe substrate, the foam treatment vessel is preferably located within apressurizable containment vessel. The substrate is submerged in asolution of carbon dioxide in deionized water under pressure, andthereafter foam is generated by reducing the pressure within thecontainment vessel. The pressure reduction causes carbon dioxide bubblesto form a foam layer on the surface of the solution. Thereafter the foamlayer is caused to pass over the substrate in moving contact therewithby discharging the solution from the foam treatment vessel.

[0031] Other objects and advantages of the invention will be apparentfrom the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a schematic diagram of a treatment apparatus forcleaning or chemically treating a semiconductor substrate, in accordancewith the invention;

[0033]FIG. 2 is a schematic diagram showing the treatment apparatus inoperation;

[0034]FIG. 3 is a schematic diagram of a chemical feed and foamgenerating system for use with the treatment apparatus of FIGS. 1 and 2;

[0035]FIG. 4 is a schematic diagram illustrating the wetting of asemiconductor substrate by a mass of foam;

[0036] FIGS. 5(a)-5(f) are schematic diagrams illustrating successivestages in the wetting of a substrate by a foam bubble;

[0037]FIG. 6 is a schematic diagram of an apparatus for supplying asolution of carbon dioxide in deionized water for drying a substrate;

[0038]FIG. 7 is a schematic diagram of an apparatus for drying asemiconductor substrate, in accordance with the invention;

[0039]FIGS. 8, 9 and 10 are schematic diagrams illustrating successivestages in the drying process using the drying apparatus of FIG. 7;

[0040] FIGS. 11-15 are schematic diagrams illustrating details of thedrying process; and

[0041]FIGS. 16 and 17 are schematic diagrams, corresponding to FIGS.8-10, illustrating final stages of the drying process.

DETAILED DESCRIPTION

[0042] The physical and chemical conditions prevailing in the cleaningof semiconductor wafers lend themselves to easy and simple foamingperformance optimization. The solvent system is extremely pure water,and therefore no minerals or hardness are present to interfere withfoaming. The temperature can be readily adjusted to optimize foamingbehavior. (Foaming is always better in warmer water because the surfacetension decreases as the temperature increases.) The requiredpersistence of the foam medium is generally less than one or twominutes. Accordingly, it is possible to use very fast-draining foams. Infact the use of fast-draining foams is desirable so that the foamself-destructs, avoiding the need for additional measures to remove it.

[0043] Foams are metastable and are formed by adding energy to agas/liquid combination. As soon as the agitation, or mixing, force isremoved the foam will start draining, thereby providing a liquid phaseessentially identical to that provided in a condensed phase, all liquid,system. Therefore, for any liquid system shown to be an effectivecleaning medium, a corresponding foam system will perform identically,because the surface layer composition, next to the semiconductor wafer,will be the same in both cases.

[0044] The significance of the use of foam in semiconductor surfacecleaning technologies is that a foam-based cleaning system is capable ofremoving more particles than it generates. In contrast, with a condensedphase liquid system, the particle count potentially increases insuccessive processing steps, yielding potentially unsatisfactory finalresults.

[0045] The apparatus depicted in FIG. 1 is used for cleaning andchemically treating a substrate. It comprises an inner vessel 18 havinga top opening 20, located within an outer vessel 22, which is also openat the top. A substrate 24 is supported within vessel 18 on a supportingframe 26. The substrate is typically a semiconductor substrate such as asilicon wafer, either in the raw state or at one of the many stages inthe fabrication process. Although only one wafer is shown, it should beunderstood that, in a wafer cleaning or chemical treatment operation,the support frame may carry a large number of wafers.

[0046] A foam inlet line 28 is connected to the interior of vessel 18 ator near the bottom of the vessel. A discharge line 30 is connected tothe bottom of the outer vessel 22 through a valve 32, and to the bottomof the inner vessel 18 through valves 32 and 34 in series.

[0047] The foam inlet line 28 carries foam into the inner vessel from astatic mixer 36 shown in FIG. 3. The static mixer can be any of avariety of known devices used to produce foam by mixing a gas with aliquid containing a surface tension-reducing agent. Suitable staticmixers are described in U.S. Pat. No. 4,400,220, dated Aug. 23, 1983 andU.S. Pat. No. 5,133,991, dated Jul. 28, 1992. A suitable surfacetension-reducing agent is a nonionic surfactant available from WakoChemical of Richmond, Va. under number NCW-601A. A wide variety ofsubstances can be used as surface-tension reducing agents for thepurpose of forming foams in deionized water. These include conventionalsurfactants, i.e. soaps and detergents of course, but also include awide range of other substances such as isopropyl alcohol, nitrous oxide,isobutane, and carbon dioxide. The concentration of the surfacetension-reducing agent should be sufficiently low to provide a fastdrain time since, as will be apparent, it is important to avoid fillingthe outer vessel with foam. In the case of conventional surfactants, theappropriate concentration is typically in the range of 100 ppm. to 2000ppm. For isopropyl alcohol, the concentration will typically be in therange of 1.0. Various other solutes, including reactants such asammonium hydroxide, nitric acid, hydrofluoric acid, etc. are normallyused in concentrations too low to produce effective foams, and need tobe supplemented by other surface tension reducing agents such asconventional surfactants.

[0048] As shown in FIG. 3, a gas supply line 38 delivers a gas to thestatic mixer 36 and also to liquid supply tanks 40, 42, 44, 46 and 48.The gas pressure is thus used to drive liquids from the tanks into thestatic mixer. The gas can be any of many suitable gases, includingnitrogen, argon, air, carbon dioxide and other gases. In general,nitrogen is preferred.

[0049] Tank 40 contains a solution consisting of a surfacetension-reducing agent in deionized water. Tanks 42-48 hold a variety ofreactants used for chemical treatment of the substrates. Examples ofsuch reactants are cleaning mixtures such as solutions of ammoniumhydroxide and hydrogen peroxide in deionized water, and etchants such assolutions of hydrofluoric acid in water. Valves 50, 52, 54 and 56control the flow of reactants from the reactant supply tanks so that thereactants can be supplied selectively to the static mixer along with thesolution of surface tension-reducing agent.

[0050] As shown in FIG. 2, foam is introduced through the foam inletline 28 until the inner vessel 18 is filled, and then continuedintroduction of foam causes excess foam to overflow the inner vessel anddrop into the outer vessel. The foam is relatively short-lived, and thelayer 60 in the lower portion of the outer vessel 22 quickly drains,forming a layer 62 of liquid, which is carried off though valve 32 todischarge line 30.

[0051] The introduction of foam into vessel 18 causes the foam withinthe vessel to rise continuously so that it passes over the substrate 24in moving contact with the substrate.

[0052] As shown in FIG. 4, the foam bubbles 64 adjacent the surface 66of the substrate are draining. As shown in FIG. 5(a), when a bubble 68approaches the surface 66, and the bubble has not yet wetted thesurface, the surface is dry. However, as shown in FIG. 5(b), as soon asthe bubble contacts the surface, it almost immediately wets the surfaceover an area approximately equivalent to the cross-section of theoriginal bubble through its center. FIGS. 5(c)-5(f) show that, as thebubble drains, the wetted area gradually increases until the bubble isfully drained as in FIG. 5(f). The mass of bubbles, as shown in FIG. 4,forms a continuous film on the surface 64 of the substrate.

[0053] As the mass of bubbles moves across the surface of the substrate,particles are scrubbed from the surface. Thus, if the foam consists onlyof deionized water and surface tension-reducing agent, it serves toclean the substrate. On the other hand, if the foam also includes areactant, it not only removes any remaining particles from the surface,but also delivers a film of reactant to the surface. In both cases, thequantity of particles delivered to the surface of the substrate by thefoam itself is far lower than the quantity that would be delivered by aliquid in a cleaning or chemical treatment operation.

[0054] The fast drain time of the foam is important not only in order toprevent the outer vessel from filling with foam, but also in order toallow the reactant to be applied to the substrate in a liquid phase.

[0055] While in the treatment vessel 18, the substrate can be exposed toa series of cleaning and chemical treating steps. For example, thesubstrate may be cleaned by first introducing a foam consistingessentially of a surface tension-reducing agent and deionized water.Thereafter, by opening the appropriate valve in the chemical feed andfoam generating system of FIG. 3, a foam comprising a surfacetension-reducing agent, deionized water and a reactant is introduced inorder to effect a chemical reaction, e.g. chemical cleaning, on thesurface of the substrate. Thereafter, the flow of the reactant is cutoff, and the reactant is rinsed from the surface of the substrate byfoam once again consisting essentially of the surface tension-reducingagent and deionized water. The rinsing step may be followed by anotherchemical treatment step, e.g. etching, carried out by opening anappropriate valve in FIG. 3 to introduce an etchant into the staticmixer along with the surface tension-reducing agent solution so that thefoam film in contact with the substrate applies the etchant to thesubstrate. Chemical treatment and rinsing steps may be carriedalternately in an extended sequence of steps in which the reactants aredifferent in the successive reactant introduction steps.

[0056] The use of foam not only reduces the quantity of particlesdelivered to the surface of the substrate in the cleaning and chemicaltreatment process, but also reduces the quantity of reactants needed tocarry out chemical treatment.

[0057] The drying process utilizes a carbon dioxide solution indeionized water. Carbon dioxide has a number of desirable properties,which make it ideal for use in a foam drying process. It is inexpensive,readily available, water soluble, and non-flammable. It is alsonon-toxic, causes no VOC emissions, and a water solution of carbondioxide can be disposed of in a conventional sanitary sewer withoutspecial treatment or other precautions. Although a solution of carbondioxide in water forms carbonic acid (H₂CO₃), the level of carbonic acidformed is very low, and it has little, if any, effect on the pH of thesolution. In water, carbon dioxide serves as a surface tension reducingagent, thereby allowing foaming.

[0058] The carbon dioxide solution is generated by the apparatus shownin FIG. 6. Carbon dioxide is maintained under pressure in a tank 70,which is refilled from time to time through a valve 72 in inlet line 74.In tank 70, carbon dioxide is present in the liquid phase at 76 and inthe gaseous phase at 78. The gaseous carbon dioxide is fed, throughvalve 80 and check valve 82, to a tank 84, where it is dissolved indeionized water to form a solution 86. The solution has a layer 88 ofgaseous carbon dioxide above it, which is under pressure. The pressureof this layer of gaseous carbon dioxide is used to discharge thesolution, through line 90 and valve 92, to a drying apparatus. Deionizedwater is replenished through line 94 and valve 96, and the solution iskept in motion by agitator 98 to maintain homogeneity.

[0059] The carbon dioxide solution is delivered through line 90 to theapparatus shown in FIGS. 7-10. The apparatus of FIGS. 7-10 can be thesame apparatus as depicted in FIG. 1, but is provided with a top closure100, allowing the outer vessel to be pressurized, a pressure controlvalve 102 for controlled venting of pressure within the outer vessel,and a valve 104, which can be closed after the carbon dioxide solutionis fed into the inner vessel 18.

[0060] In the drying operation, the inner vessel 18 is filled withcarbon dioxide solution through valve 104 while the pressure controlvalve 102 is either closed, or only partly opened, in order to preventcarbon dioxide bubbles from being released from the solution as it fillsthe inner vessel 18. As shown in FIG. 8, the carbon dioxide solutionfills the inner vessel to a level 106, above the uppermost part of thesubstrate 24.

[0061] After filling the inner vessel 18, valve 102 is opened in acontrolled manner to relieve the pressure within the outer vessel. Therelieving of the pressure causes carbon dioxide bubbles to be releasedfrom the solution. These bubbles form a layer 108 of foam on the surfaceof the solution as shown in FIG. 9.

[0062] Promptly after the layer of foam is formed, valves 34 and 32 areopened to discharge the carbon dioxide solution from the inner vesselthrough line 30. The discharge takes place by gravity, but may beassisted by residual gas pressure within the enclosure. The foam layer108 descends, as shown in FIG. 10, passing downwardly over thesubstrate.

[0063] As shown in FIGS. 11-15, the foam layer scrubs particles from thesurfaces of substrate 24 as it descends. In FIG. 11, the substrate iscompletely submerged in the carbon dioxide solution. Particles 110 areshown adhering to the surfaces of the substrate. As shown in FIGS. 12and 13, the particles 110 are scrubbed by the interface of the foamlayer and the carbon dioxide solution, and carried downward by the foamlayer along the surfaces of the substrate so that they are removed fromthe substrate as shown in FIG. 14 and carried toward the bottom of thevessel.

[0064] As the solution continues to be discharged from the vessel, thefoam layer 108 clears the substrate, leaving the substrate in anatmosphere of carbon dioxide with some water vapor, as shown in FIG. 16.Ultimately, the foam layer 108 collapses leaving a residue 112 as shownin FIG. 17.

[0065] An example of a typical drying operation in accordance with theinvention is as follows: The process starts with a frame carrying fifty200 mm wafers, with 6 mm spacing, submerged in an overflow tank, i.e.vessel 18 in FIG. 8, within the drying unit containment vessel, i.e.vessel 22. The overflow tank contains, pure deionized water with eitherair or nitrogen as the overhead gas at atmospheric pressure or slightlyabove. The tank size is 25 cm (depth)×25 cm (width)×50 cm (length).Therefore, the volume is 31.25 liters without considering the volume ofthe carrying frame or the wafers. The wafers are about 100 cm³ each, or5000 cm³ (5 liters) total, and the carrying frame is 1000 cm³ (1 liter).Therefore, the liquid volume is 25.25 liters.

[0066] The containment vessel includes the necessary valves, checkvalve, pressure relief valves, baffles, channels, inlets, outlets, andaccess panels (doors, lids, etc.) to allow the process to operate safelyat a total pressure of approximately 300 psig. To simplify calculations,assume that the total volume of the containment vessel is 30.00 liters.The external containment vessel is closed and prepared forpressurization.

[0067] The surface tension of CO₂ in water at 300 psig total pressure isbetween 57 dynes/cm at 11° C. and 59 dynes/cm at 45° C. This suggeststhat temperature is not an important variable, but that is misleading,since the solubility of CO₂ in water at 300 psig is very dependent uponthe temperature. At 300 psig and 10° C. the solubility of CO₂ in waterwill produce a 4.5 weight percent solution. Since the CO₂ not onlyreduces the surface tension, but also generates the foam, the amount ofCO₂ in the water is important, since the foam must be maintained duringthe liquid discharge interval.

[0068] In a separate adjacent vessel, suitably designed and equipped,and plumbed to the drying unit described above, pure deionized water issaturated with carbon dioxide at a pressure slightly higher than the 300psig design pressure, and at a temperature about 10° C., yielding aconcentration of carbon dioxide in water at 4.5 weight percent. Thereason for the slight pressure increase is that the liquid will bepressure fed into the drying vessel, thereby eliminating the need forpumping systems which, in general, are sources of contamination due totheir moving parts.

[0069] The pure deionized submersion water is replaced by equally puredeionized water containing carbon dioxide at 10° C. The atmospheric gasor nitrogen in the containment vessel is discharged and replaced withCO₂ while, at the same time, the total pressure of the system isincreased to 300 psig. This step can be executed either of two ways. Thedissolved CO₂ in the water can exchange with the air or nitrogenatmosphere, forming a new pressurized atmosphere of CO₂. Alternatively,the overhead atmospheric gas is displaced by direct injection of CO₂,purging the air or nitrogen while increasing the pressure within thecontainment vessel. The latter approach is desirable, because it isfaster and produces less waste.

[0070] It is not necessary to eliminate every molecule of the originaloverhead gas. In general three displacement volumes are sufficient. Thegas volume in the containment vessel is 30000 cm³, which converts to120,000 cm³ CO₂ measured at STP (ignoring minor temperature differences)Since 44 grams of CO₂ occupy 22400 cm³ at STP, the weight of CO₂ usedis:

44×120000/22400=236 grams.

[0071] The pressure must be elevated to 20 atm, 300 psig, thereforerequiring twenty times more CO₂:

236×20/1=4720 grams.

[0072] This procedure removes the overhead gas and pressurizes thecontainment vessel. The time interval required to carry out thisoperation is only about one minute, and therefore the amount of CO₂adsorbed by the submersion water is not significant.

[0073] The next step is to displace the original submersion water. Thecarbon dioxide-saturated water in the storage tank is pressure fed intothe containment vessel by a near quantitative displacement. It is notcritical if some of the original submersion water is retained. This steprequires 25.25 liters of water, 25250 grams, containing 4.5 wt % CO₂,or,

22250×0.045=1136 grams of CO₂.

[0074] The wafers are now submerged in a solution of carbondioxide-saturated, deionized water. The surface tension of the solutionis 57-59 dynes/cm. The overhead gas is carbon dioxide at 300 psig.

[0075] Quiescent, supersaturated solutions of carbon dioxide in waterdepend upon external forces to initiate the desorption process. Here,however, the carbon dioxide solution has no opportunity to becomequiescent if, promptly after the carbon dioxide solution is introduced,depressurization of the outer vessel occurs. Depressurization initiatesfoaming, and the introduction of new carbon dioxide solution while theliquid level drops causes foaming to continue. Other measures, such asagitation, imparting turbulence to the incoming carbon dioxide solution,or the introduction of small amounts of nitrogen or other gas, can beused to ensure initiation of foaming, where necessary.

[0076] For the next step, assume that the carbon dioxide foam providesan expansion ratio of ten (E/R=10); a 100% drain time of 20 seconds; andan average foam thickness on the surface of 25.4 mm. Thefluid-atmosphere interface descent rate should be about 50-75 mm/minute.If the 250 mm depth of the overflow vessel is covered in 4 minutes, theaverage rate is 64 mm/minute.

[0077] The surface of the overflow tank is 200 in², so the volume of thefoam required is 200 in³ (3277 cm³). The foam volume has to bemaintained for the four minutes required for the venting of thecontainment vessel and the descent of the gas-liquid interface.

[0078] The venting can proceed linearly, and programmed depressurizationcan be accomplished by the declining liquid level and controlledoverhead gas venting.

[0079] Since the foam volume needs to be maintained, the amount ofcarbon dioxide delivered to the drying unit has to be constant, butadjusted for the declining pressure. At 20 atmospheres, the carbondioxide requirement is twenty times higher than at 1 atmosphere. Theonly source of carbon dioxide during this process interval is thesaturated water solution stored in the adjacent tank.

[0080] The liquid level in the tank declines, but as this takes place,carbon dioxide and water are added so that the foaming of the carbondioxide can maintain the foam blanket. Therefore, the carbondioxide-water flow rate is defined by the amount of carbon dioxiderequired, while the liquid discharge rate from the vessel mustaccommodate the original liquid plus the added influent, while stillmaintaining a proper gas-liquid interface descent rate.

[0081] The system starts at 300 psig (315 psia) and declines to 0 psig(15 psia) in four minutes.

[0082] The following Table I, illustrates the conditions under which aconstant volume of foam can be maintained as the liquid is discharged.

[0083] Column D lists the foam volume, which must be replaced duringeach 20-second interval. The foam volume is constant and is equal to thehorizontal cross sectional area of the foam layer multiplied by itsheight. Column E collects the cumulative foam volume.

[0084] In Column F, the carbon dioxide volume portion of the foam, atpressure, is given. The expansion ratio of the foam is ten. Therefore,90% of the foam volume is carbon dioxide, the expansion gas, while thebalance is water. Column G accumulates Column F.

[0085] Column H converts the Column F data to carbon dioxide volume(cc.) at STP, without temperature adjustment. The pressure conversion ismeasured in absolute pressure, not gauge pressure, so the factor is:(15+P)/15. Column I is the cumulative carbon dioxide volume at STP incubic centimeters.

[0086] Column J converts Column H into carbon dioxide by weight, using22400 cc/mole as the standard molar volume and 44, the molecular weightof carbon dioxide. Column K is the cumulative carbon dioxide weight.

[0087] Columns L, M, N, and O show similar data for the water portion ofthe incoming feed stream. Since the carbon dioxide is 4.5 weight %, thewater portion must be 95.5 weight %. Column L displays the 20-secondinterval data in grams, while Column M shows the cumulative data,converted to kilograms, without the carbon dioxide portion included.TABLE I DRYING VESSEL DISCHARGE DATA PART 1 F G H I D E INTERVALCUMULATIVE INTERVAL CUMULATIVE A B C INTERVAL CUMULATIVE CO2 GAS CO2 GASCO2 GAS CO2 GAS TIME TIME PRESSURE FOAM FOAM at P at P STP STP (sec)(min) (psig) (cc) (cc) (cc) (cc) (cc) (cc) 1 0 0.00 300 3277 3277 29492949 61935 61935 2 20 0.33 275 3277 6554 2949 5899 57020 118955 3 400.67 250 3277 9831 2949 8848 52104 171059 4 60 1.00 225 3277 13108 294911797 47189 218248 5 80 1.33 200 3277 16385 2949 14747 42273 260522 6100 1.67 175 3277 19662 2949 17696 37358 297879 7 120 2.00 150 327722939 2949 20645 32442 330322 8 140 2.33 125 3277 26216 2949 23594 27527357848 9 160 2.67 100 3277 29493 2949 26544 22611 380460 10 180 3.00 753277 32770 2949 29493 17696 398156 11 200 3.33 50 3277 36047 2949 3244212780 410936 12 220 3.67 25 3277 39324 2949 35392 7865 418801 13 2404.00 0 3277 42601 2949 38341 2949 421750 PART 2 J K N O INTERVALCUMULATIVE L M INTERVAL INTERVAL A B C CO2 GAS CO2 GAS INTERVALCUMULATIVE WATER WATER TIME TIME PRESSURE WEIGHT WEIGHT WATER WATER FLOWFLOW (sec) (min) (psig) (gm) (gm) (gm) (kgm) (gm/sec) (gpm) 1 0 0.00 300122 122 2704 2.70 141.3 2.24 2 20 0.33 275 112 234 2489 5.19 130.0 2.063 40 0.67 250 102 336 2274 7.47 118.8 1.88 4 60 1.00 225 93 429 20609.53 107.6 1.71 5 80 1.33 200 83 512 1845 11.37 96.4 1.53 6 100 1.67 17573 585 1631 13.00 85.2 1.35 7 120 2.00 150 64 649 1416 14.42 74.0 1.17 8140 2.33 125 54 703 1202 15.62 62.8 1.00 9 160 2.67 100 44 747 987 16.6151.6 0.82 10 180 3.00 75 35 782 772 17.38 40.4 0.64 11 200 3.33 50 25807 558 17.94 29.1 0.46 12 220 3.67 25 15 823 343 18.28 17.9 0.28 13 2404.00 0 6 828 129 18.41 6.7 0.11

[0088] As will be apparent from columns N and O, the water flowratedecreases linearly over time.

[0089] Ultimately, the drying vessel depressurization is complete andthe vessel is empty, closed, and at atmospheric pressure. The atmospherein the vessel is essentially all carbon dioxide as the vapor pressure ofwater at 10° C. is about 10 mm Hg., thereby defining that the partialpressure of carbon dioxide is 750 mm Hg. when the total pressure is oneatmosphere or 760 mm Hg.

[0090] The entire vessel with its wafers and the supporting carriagewill have some small amount of water remaining, particularly at thecontact points where the wafers touch the carrying frame. That water,and any other within the vessel, can be removed by using carbon dioxideas a purging gas at room temperature for a few minutes, with the finalcarbon dioxide replaced with nitrogen, if desired. The drycarrier-supported wafers can then be removed from the drying equipmentand stored or transported in an appropriate enclosure.

[0091] The following example illustrates a flow-through processanalogous to the current, continuous phase, flow-through process inwhich deionized water is the medium and active ingredients are added asthe chemical treatment process progresses. The treatment process beginswith water, followed by etching, oxidizing, neutralization, each suchstep being followed by an intervening rinsing step, and finally dryingat the end of the sequence.

[0092] In accordance with the invention, the same general sequence isexecuted using a foam medium. The expansion gas can be argon, air, ornitrogen, but nitrogen is preferred.

[0093] Instead of deionized water, the initial medium is deionized waterwith Wako Chemical NCW-601A surfactant, added as a foaming agent. Thelevel of surfactant needs to be low, e.g. 300 ppm, enough to provide afast drain time.

[0094] The wafers are placed in a flow-though system corresponding tothe apparatus of FIG. 1, and foam is passed over them. In the firststep, treatment with ammonium hydroxide and hydrogen peroxide, the“to-be-foamed” liquid (deionized water and surface tension-reducingagent) has the required amount of ammonium hydroxide and peroxide added.Foaming occurs, and the wafers are treated with this oxidizing solution.It is important that the foam has a fast drain time, as it must producea liquid phase of the desired composition. For example, if the desiredsolution requires 1000 ppm of hydrogen peroxide and 500 ppm of ammoniumhydroxide, plus surface tension-reducing agent, in deionized water, thenthat solution is composed, foamed, and injected into the treatmentvessel. The appropriately timed drainage provides the liquid phasecomposition to the surface of the wafer, but reduces the total volume ofmaterial required in proportion to the reciprocal of the expansionratio.

[0095] After five minutes of treatment time, the hydrogen peroxide andthe ammonium hydroxide injections are stopped, but the surfacetension-reducing agent and deionized water continue. The foaming phaseis now a rinsing phase utilizing only deionized water and surfacetension-reducing agent. This rinsing foam continues to pass through thetreatment vessel until the previous reactants are flushed away.

[0096] The next sequence is an etching cycle, and the appropriateingredients are added to the liquid before foaming, foaming occurs,treatment follows, the actives injection is stopped, and the rinsingcycle follows.

[0097] The third step follows in the same manner, as does the fourth,and fifth, if required, each ending with a rinsing phase.

[0098] Eventually, the chemical treatment is completed, and dryingbecomes the final step. The injection of surface tension-reducing agentinto the water is stopped, the expansion gas injection is stopped, thefoaming stops, and the system is converted to a liquid system, while theresidual surface tension-reducing agent is flushed from the treatmentvessel. This leaves the treatment vessel filled with deionized water,and the drying process can proceed. Carbon dioxide-saturated water, asdescribed above, is injected into the now pressurized vessel, and thedrying sequence continues.

[0099] It is important to recognize that all of the process variables,including fluid temperature and fluid composition, can be accommodatedwith a foam based system, simply by heating or cooling the incomingdeionized water and injecting the proper levels of ingredients.

[0100] Various modifications can be made to the apparatus and processesdescribed. For example, the cleaning and chemical treatments can takeplace either in the same vessels or in separate vessels, and the dryingcan take place in the same vessel in which cleaning and chemicaltreatment take place. Normally, the cleaning and chemical processingsteps take place sequentially without any intermediate drying steps, anddrying is carried out only as a final step when the wafers are to beremoved from the treatment vessel. However, cleaning and chemicaltreatment steps may be carried out alternately with drying in the samevessel, especially when one or more of the chemical treatment steps isgas treatment.

[0101] Furthermore, as indicated previously, in the drying process,carbon dioxide can be introduced to purge the air, nitrogen or otheratmosphere by introduction of carbon dioxide solution rather than bydirect carbon dioxide injection. Still other modifications can be madewithout departing from the scope of the invention as defined by thefollowing claims.

1. A process for treatment of a semiconductor substrate having a surfaceto which undesired particles adhere comprising: moving a foam and thesemiconductor substrate relative to each other, thereby causing the foamto pass over the surface of the semiconductor substrate in movingcontact therewith; and carrying away undesired particles from saidsurface of the substrate with the foam.
 2. The process according toclaim 1, in which the foam and substrate are caused to move relative toeach other by the introduction of foam into a treatment vessel in whichthe substrate is situated.
 3. The process according to claim 1, in whichthe foam and substrate are caused to move relative to each other by acontinuous introduction of foam into, and discharge of the foam from, atreatment vessel in which the substrate is situated.
 4. The processaccording to claim 1, in which the liquid component of the foam consistsof a surface tension-reducing agent and deionized water.
 5. The processaccording to claim 1, in which, the liquid component of the foamcomprises a surface tension-reducing agent, deionized water and at leastone reactant for chemical treatment of the semiconductor substrate,whereby a chemical reaction takes place between the substrate and thereactant.
 6. The process according to claim 1, in which the foam andsubstrate are caused to move relative to each other by the introductionof foam into a treatment vessel in which the substrate is situated, andin which the introduction of foam into the treatment vessel is carriedout by first introducing into the treatment vessel a foam consistingessentially of a surface tension-reducing agent and deionized water, andthereafter introducing into the treatment vessel a foam comprising asurface tension-reducing agent, deionized water and at least onereactant for chemical treatment of the substrate whereby a chemicalreaction takes place between the substrate and the reactant.
 7. Theprocess according to claim 1, in which the foam and substrate are causedto move relative to each other by the introduction of foam into atreatment vessel in which the substrate is situated, in which theintroduction of foam into the treatment vessel is carried out by firstintroducing a foam consisting essentially of a surface tension-reducingagent and deionized water, thereafter introducing a foam comprising asurface tension-reducing agent, deionized water and at least onereactant for chemical treatment of the substrate whereby a chemicalreaction takes place between the substrate and the reactant, andthereafter introducing a foam consisting essentially of a surfacetension-reducing agent and deionized water, whereby said at least onereactant is rinsed from the substrate.
 8. The process according to claim1, in which the foam and substrate are caused to move relative to eachother by the introduction of foam into a treatment vessel in which thesubstrate is situated, in which the introduction of foam into thetreatment vessel is carried out by alternately introducing a foamconsisting essentially of a surface tension-reducing agent and deionizedwater, and a foam comprising a surface tension-reducing agent, deionizedwater and at least one reactant for chemical treatment of the substrate.9. The process according to claim 1, in which the semiconductorsubstrate is situated in a treatment vessel, in which foam is introducedinto the treatment vessel, and in which foam in contact with the surfaceof the semiconductor substrate is caused to move relative to the surfaceof the semiconductor substrate as a result of its displacement by foamintroduced into the foam treatment vessel.
 10. The process according toclaim 1, in which the semiconductor substrate is situated in a treatmentvessel, in which foam is introduced into the treatment vessel, in whichfoam in contact with the surface of the semiconductor substrate iscaused to move relative to the surface of the semiconductor substrate asa result of its displacement by foam introduced into the foam treatmentvessel, and in which a sufficient quantity of foam is introduced to fillthe treatment vessel and thereafter, by continued introduction of foaminto the treatment vessel, foam carrying undesired particles is causedto discharge from the treatment vessel.
 11. A process for treatment of asemiconductor substrate having a surface to which undesired particlesadhere comprising: generating a foam consisting of gas bubbles and aliquid component; and by moving the foam and the semiconductor substraterelative to each other while the foam is in contact with said surface,causing the foam to carry said undesired particles away from saidsurface of the substrate.
 12. The process according to claim 11, inwhich the foam and substrate are caused to move relative to each otherby the introduction of foam into a treatment vessel in which thesubstrate is situated.
 13. The process according to claim 11, in whichthe foam and substrate are caused to move relative to each other by acontinuous introduction of foam into, and discharge of the foam from, atreatment vessel in which the substrate is situated.
 14. The processaccording to claim 11, in which the liquid component of the foamconsists of a surface tension-reducing agent and deionized water. 15.The process according to claim 11, in which, the liquid component of thefoam comprises a surface tension-reducing agent, deoinized water and atleast one reactant for chemical treatment of the semiconductorsubstrate, whereby a chemical reaction takes place between the substrateand the reactant.
 16. The process according to claim 11, in which thefoam and substrate are caused to move relative to each other by theintroduction of foam into a treatment vessel in which the substrate issituated, and in which the introduction of foam into the treatmentvessel is carried out by first introducing into the treatment vessel afoam consisting essentially of a surface tension-reducing agent anddeionized water, and thereafter introducing into the treatment vessel afoam comprising a surface tension-reducing agent, deionized water and atleast one reactant for chemical treatment of the substrate whereby achemical reaction takes place between the substrate and the reactant.17. The process according to claim 11, in which the foam and substrateare caused to move relative to each other by the introduction of foaminto a treatment vessel in which the substrate is situated, in which theintroduction of foam into the treatment vessel is carried out by firstintroducing a foam consisting essentially of a surface tension-reducingagent and deionized water, thereafter introducing a foam comprising asurface tension-reducing agent, deionized water and at least onereactant for chemical treatment of the substrate whereby a chemicalreaction takes place between the substrate and the reactant, andthereafter introducing a foam consisting essentially of a surfacetension-reducing agent and deionized water, whereby said at least onereactant is rinsed from the substrate.
 18. The process according toclaim 11, in which the foam and substrate are caused to move relative toeach other by the introduction of foam into a treatment vessel in whichthe substrate is situated, in which the introduction of foam into thetreatment vessel is carried out by alternately introducing a foamconsisting essentially of a surface tension-reducing agent and deionizedwater, and a foam comprising a surface tension-reducing agent, deionizedwater and at least one reactant for chemical treatment of the substrate.19. The process according to claim 11, in which the semiconductorsubstrate is situated in a treatment vessel, in which foam is introducedinto the treatment vessel, and in which foam in contact with the surfaceof the semiconductor substrate is caused to move relative to the surfaceof the semiconductor substrate as a result of its displacement by foamintroduced into the foam treatment vessel.
 20. The process according toclaim 11, in which the semiconductor substrate is situated in atreatment vessel, in which foam is introduced into the treatment vessel,in which foam in contact with the surface of the semiconductor substrateis caused to move relative to the surface of the semiconductor substrateas a result of its displacement by foam introduced into the foamtreatment vessel, and in which a sufficient quantity of foam isintroduced to fill the treatment vessel and thereafter, by continuedintroduction of foam into the treatment vessel, foam carrying undesiredparticles is caused to discharge from the treatment vessel.